1. Field of the Invention
The present invention relates to spherical boron nitride nanoparticles which have nano meter size diameter, and synthetic method thereof.
2. Description of the Related Art
Some methods for synthesizing boron nitride particles are known, for example, high-temperature syntheses described in J. Mater. Sci. Lett., vol. 16, 795 (1997) and Ceram. Sci. Eng. Proc., vol. 6. 1305 (1985); chemical vapor deposition synthesis described in Thin Solid Films, vol. 157. 267 (1988); and polymer calcinating synthesis described in Inorg. Chem., vol. 5. 989 (1966). In particular, a method of reacting boron halogenide with ammonia, described in EP396448 A is well known. In this reaction, boron trihalide and ammonia are heated at low temperature for synthesizing boron nitride particles. Lindquist et al disclosed a method for synthesizing spherical boron nitride nanoparticles by calcinating polyborazinylamine which is dissolved in liquid ammonia; see J. Am. Ceram. Soc., vol. 74. 3126. (1994). Recently a method of spherical boron nitride particles having average diameter of 1-2 micrometers were reported. In this method the spherical boron nitride nanoparticles were synthesized from boric acid and ammonia by reacting at high temperature; see Chem. Mater., vol. 12, 19 (2000). Synthetic methods for boron nitride nanotube are disclosed, for example, in JP 2000-109306 A, JP2001-270707 A, and JP 2002-097004 A, although they are not spherical particles of boron nitride.
Moreover, a method for synthesizing spherical boron nitride nanoparticles having average diameter of 100-180 nm was disclosed in JP 1985-200811 A. In this method, carbon and boron oxide obtained by hydrolysis of borate ester are heated in nitrogen-containing compound atmosphere to synthesize spherical boron nitride nanoparticles having average diameter of 100-180 nm. Furthermore a method for synthesizing spherical boron nitride nanoparticles having average diameter of 50-100,000 nm was disclosed in U.S. Pat. No. 6,824,753. The method comprises a first step of heating organic boron precursor, inactive gas and nitriding agent at 600-1800 degrees C. to obtain powder of BNxOyCz; and a second step of heating the obtained powder and further added nitriding agent at 600-1800 degrees C. Also the inventors filed a patent application about a method for synthesizing spherical boron nitride nanoparticles having submicron size and high purity from trimethyl borate and ammonia. The obtained spherical boron nitride nanoparticles contained a little amount of oxygen, see JP 2004-182572 A. However, above references did not be carried out study on manufacturing conditions and detailed method, and diameter of obtained spherical boron nitride nanoparticles were large size. Accordingly, in the case spherical boron nitride nanoparticles having large diameter were used in a composite material, as fillers, the thermal conductivity was not improved well enough. On the other hand, it is not in the same technical field of spherical boron nitride nanoparticles, but some reports showed different crystal morphologies, for example nano belts or nano rods. The reports suggest possibility of new materials having new form and/or small diameter if manufacturing conditions and/or detailed method were studied in detail, see Adv. Funct. Mater., vol. 15. No. 1, vol. 63, 2005.